Oligonucleotides are known to hybridize to single-stranded DNA or RNA molecules. Hybridization is the sequence-specific base pair hydrogen bonding of nucleobases of the oligonucleotides to nucleobases of target DNA or RNA. Such nucleobase pairs are said to be complementary to one another.
The complementarity of oligonucleotides has been used for inhibition of a number of cellular targets. Such complementary oligonucleotides are commonly described as being antisense oligonucleotides. Various reviews describing the results of these studies have been published including Progress In Antisense Oligonucleotide Therapeutics, Crooke, S. T. and Bennett, C. F., Annu. Rev. Pharmacol. Toxicol., 1996, 36, 107-129. These oligonucleotides have proven to be very powerful research tools and diagnostic agents. Further, certain oligonucleotides that have been shown to be efficacious are currently in human clinical trials.
To date most oligonucleotides studied have been oligodeoxynucleotides. Antisense oligodeoxynucleotides are believed to cause a reduction in target RNA levels principally through the action of RNase H, an endonuclease that cleaves the RNA strand of DNA:RNA duplexes. This enzyme, thought to play a role in DNA replication, has been shown to be capable of cleaving the RNA component of the DNA:RNA duplexes in cell free systems as well as in Xenopus oocytes. Rnase H is very sensitive to structural alterations in antisense oligonucleotides. This sensitivity is such that prior attempts to increase the potency of oligonucleotides by increasing affinity, stability, lipophilicity and other characteristics by chemical modifications of the oligonucleotide have often resulted in oligonucleotides that are no longer substrates for Rnase H. In addition, Rnase H activity is quite variable. Thus a given disease state may not be a candidate for antisense therapy only because the target tissue has insufficient Rnase H activity. Therefore it is clear that effective terminating mechanisms in addition to Rnase H are of great value to the development of therapeutic and other agents.
Several publications describe the interaction of Rnase H and oligonucleotides. A recently publication is: Crooke, et. al., Biochem. J., 1995, 312, 599-608. Other earlier papers are: (1) Dagle et al., Nucleic Acids Research, 1990, 18, 4751; (2) Dagle et al., Antisense Research And Development, 1991, 1, 11; (3) Eder et al., J. Biol. Chem., 1991, 266, 6472; and (4) Dagle et al., Nucleic Acids Research, 1991, 19, 1805. According to these publications, DNA oligonucleotides having both unmodified phosphodiester internucleoside linkages and modified phosphorothioate internucleoside linkages are substrates for cellular RNase H. Since they are substrates, they activate the cleavage of target RNA by RNase H. However, these authors further noted that in Xenopus embryos, both phosphodiester linkages and phosphor-othioate linkages are also subject to exonuclease degradation. Nuclease degradation is detrimental since it rapidly depletes the oligonucleotide.
As described in references (1), (2) and (4), to stabilize oligonucleotides against nuclease degradation while still providing for RNase H activation, 2′-deoxy oligonucleotides having a short section of phosphodiester linked nucleosides positioned between sections of phosphoramidate, alkyl phosphonate or phosphotriester linkages were constructed. While the phosphoramidate-containing oligonucleotides were stabilized against exonucleases, in reference (4) the authors noted that each phosphoramidate linkage resulted in a loss of 1.6° C. in the measured Tm value of the phosphoramidate-containing oligonucleotides. Such a decrease in the Tm value is indicative of a decrease in hybridization between the oligonucleotide and its target strand.
Other authors have commented on the effect such a loss of hybridization between an oligonucleotide and its target strand can have. Saison-Behmoaras et al. (EMBO Journal, 1991, 10, 1111) observed that even though an oligonucleotide could be a substrate for Rnase H, cleavage efficiency by Rnase H was low because of weak hybridization to the mRNA. The authors also noted that the inclusion of an acridine substitution at the 3′ end of the oligonucleotide protected the oligonucleotide from exonucleases.
U.S. Pat. No. 5,013,830, issued May 7, 1991, discloses mixed oligomers comprising an RNA oligomer, or a derivative thereof, conjugated to a DNA oligomer via a phosphodiester linkage. The RNA oligomers also bear 2′-O-alkyl substituents. However, being phosphodiesters, the oligomers are susceptible to nuclease cleavage.
European Patent application 339,842, published Nov. 2, 1989, discloses 2″-O-substituted phosphorothioate oligonucleotides, including 2′-O-methylribooligonucleotide phosphorothioate derivatives. The above-mentioned application also discloses 2′-O-methyl phosphodiester oligonucleotides which lack nuclease resistance.
U.S. Pat. No. 5,149,797, issued Sep. 22, 1992, discloses mixed phosphate backbone oligonucleotides which include an internal portion of deoxynucleotides linked by phosphodiester linkages, and flanked on each side by a portion of modified DNA or RNA sequences. The flanking sequences include methyl phosphonate, phosphoromorpholidate, phosphoropiperazidate or phosphoramidate linkages.
U.S. Pat. No. 5,256,775, issued Oct. 26, 1993, describes mixed oligonucleotides that incorporate phosphoramidate linkages and phosphorothioate or phosphorodithioate linkages.
U.S. Pat. No. 5,403,711, issued Apr. 4, 1995, describes RNA:DNA probes targeted to DNA. The probes are labeled and are used in a system that includes RNase H. The RNase H enzyme cleaves those probes that bind to DNA targets. The probes can include modified phosphate groups. Mentioned are phosphotriester, hydrogen phosphonates, alkyl or aryl phosphonates, alkyl or aryl phosphoramidates, phosphorothioates or phosphoroselenates.
In contrast to the pharmacological inhibition of gene expression via the RNase H enzyme, it is becoming clear that organisms from bacteria to humans use endogenous antisense RNA transcripts to alter the stability of some target mRNAS and regulate gene expression, see Nellen, W., and Lichtenstein, C., Curr. Opin. Cell. Biol., 1993, 18, 419-424 and Nellen, W., et al, Biochem. Soc. Trans. 1992, 20, 750-754. Perhaps one of the best examples comes from certain bacteria where an antisense RNA regulates the expression of mok mRNA, which is required for the translation of the cytotoxic hok protein. Thus as the antisense level drops, mok mRNA levels and consequently hok protein levels rise and the cells die, see Gerdes, K. et al., J. Mol. Biol., 1992, 226, 637-649. Other systems regulated by such mechanisms in bacteria include the RNA I-RNA II hybrid of the ColE1 plasmid, see Haeuptle, M. T., Frank, R., and Dobberstein, B., Nucleic Acids Res. 1986, 14, 1427, Knecht, D., Cell Motil. Cytoskel., 1989, 14, 92-102; and Maniak, M., and Nellen, W., Nucleic Acids Res., 1990, 18, 5375-5380; OOP-cII RNA regulation in bacteriophage Lambda, see Krinke, L., and Wulff, D. L. (1990) Genes Dev., 1990, 4, 2223-2233; and the copA-copT hybrids in E.coli. See Blomberg, P., Wagner, E. G., and Nordstrom, K., EMBO J., 1990, 9, 2331-2340. In E.coli the RNA:RNA duplexes formed have been shown to be substrates for regulated degradation by the endoribonuclease RNase III. Duplex dependent degradation has also been observed in the archaebacterium, Halobacterium salinarium, where the antisense transcript reduces expression of the early (Ti) transcript of the phage gene phiH, see Stolt, P., and Zillig, W., Mol. Microbiol., 1993, 7, 875-882. In several eukaryotic organisms endogenous antisense transcripts have also been observed. These include p53, see Khochbin and Lawrence, EMBO, 1989, 8, 4107-4114; basic fibroblast growth factor, see Volk et al, EMBO, 1989, 8, 69, 2983-2988; N-myc, see Krystal, G. W., Armstrong, B. C., and Battey, J. F., Mol. Cell. Biol., 1990, 10, 4180-4191; eIF-2α, see Noguchi et al., J. Biol. Chem., 1994, 269, 29161-29167. The conservation of endogenously expressed antisense transcripts across evolutionary lines suggests that their biological roles and molecular mechanisms of action may be similar.
In bacteria, RNase III is the double stranded endoribonuclease (dsRNase) activity responsible for the degradation of some antisense:sense RNA duplexes. RNase III carries out site-specific cleavage of dsRNA-containing structures, see Saito, H. and Richardson, C. C., Cell, 1981, 27, 533-540. The RNase III also plays an important role in mRNA processing and in the processing of rRNA precursors into 16S, 23S and 5S ribosomal RNAs, see Dunn, J. J. and Studier, F. W. J. Mol. Biol., 1975, 99, 487. In eukaryotes, a yeast gene (RNT1) has recently been cloned that codes for a protein that has homology to E.coli RNase III and shows dsRNase activity in ribosomal RNA processing, see Elela, S. A., Igel, H. and Ares, M. Cell, 1996, 85, 115-124. Avian cells treated with interferon produce and secrete a soluble nuclease capable of degrading dsRNA, see Meegan, J. and Marcus, P. I., Science, 1989, 244, 1089-1091. However such a secreted dsRNA activity is not a likely candidate to be involved in cytoplasmic degradation of antisense:sense RNA duplexes. Despite these findings almost nothing is known about human or mammalian dsRNAse activities. While it has been recognized that regulation (via any mechanism) of a target RNA strand would be useful, to date only two mechanisms for eliciting such an effect are known. These are hybridization arrest and use of an oligodeoxynucleotide to effect RNase H cleavage of the RNA target. Accordingly, there remains a continuing long-felt need for methods and compounds for regulation of target RNA. Such regulation of target RNA would be useful for therapeutic purposes both in vivo and ex vivo and, as well as, for diagnostic reagents and as research reagents including reagents for the study of both cellular and in vitro events.